Deeply fudgy, intensely chocolatey brownies enriched with reduced Guinness for roasted malt notes, finished with a creamy Guinness- and espresso-infused frosting. A decadent, grown-up chocolate dessert perfect for St. Patrick’s Day or anytime you crave rich chocolate.
In a small saucepan, bring the Guinness to a boil over medium-high heat; once boiling, reduce to medium and simmer until reduced to 2/3 cup (160 g/ml), about 12–15 minutes. Set aside to cool for at least 10 minutes.
Line a 9-inch square metal baking pan with parchment paper (do not use glass or ceramic) and set aside.
Melt the 3/4 cup (170 g) unsalted butter and the chopped semi-sweet chocolate together (microwave or stovetop) until smooth; let cool slightly.
Whisk in the granulated sugar and 1/2 cup (120 ml) of the reduced Guinness until glossy and combined.
Whisk in the eggs and vanilla until fully incorporated.
Fold in the flour, salt, and optional espresso powder just until no dry streaks remain; the batter will be thick and shiny.
Spread the batter evenly into the prepared pan and bake until the center is just set and a toothpick inserted comes out with a few moist crumbs. Cool completely in the pan.
Make the frosting: Beat the softened butter until creamy, then gradually mix in the confectioners’ sugar.
Beat in 2–3 Tablespoons of the reduced Guinness, the espresso powder, vanilla, and salt until smooth and spreadable.
Spread the frosting over the cooled brownies, slice, and serve.